Silicone rubber compositions resistant to settlement by aquatic organisms are described for example in GB-A-1307001, 1581727 and 2141436 and U.S. Pat. Nos. 3702778, 4025693 and 4525565. They have significant advantages over conventional antifouling compositions containing marine biocides which are gradually leached from the paint. The silicone rubber coatings rely on their low surface energy, rather than on biocides, to inhibit settlement of marine organisms and are referred to hereinafter as "foul resistant coatings". Conventional antifouling paints eventually become ineffective as all their biocide is released. Silicone rubber coatings are potentially capable of retaining their low energy surface indefinitely.
There are, however, practical difficulties in the use of silicone rubber foul resistant compositions, especially as coatings for ships hulls. They are mechanically rather weak; in particular they have low abrasion and tear resistance and low cohesive strength so that the coating film is easily damaged. Fouling organisms readily attach to abraded surfaces.
Silicone rubber foul resistant coatings of increased cohesive strength are described in EP-B-32597 and EP-A-89071, which describes silicone rubber coatings having grafted silicone-free segments, and in U.S. Pat. No. 5017322 which describes a simultaneously cured coating of silicone rubber and polyurethane or polyurea, consisting of domains of polyurethane or polyurea within a network of crosslinked silicone elastomer. There is a need for foul resistant coatings with greater abrasion resistance than can be achieved using such coatings. The proportion of silicone-free segments or of polyurethane domains which can be included in the silicone rubber whilst maintaining resistance to fouling is limited.
WO-80/00554 describes the prevention of fouling of a marine structure such as an oil production platform or drilling rig by securing to the underwater surface a coated flexible sheet material, the outermost surface of which is a layer of silicone rubber. The preferred sheet material is a coated fabric. Such a cladding may be feasible for static structures but is less practical for the hulls of ships and other marine craft.
Undercoats for silicone rubber foul resistant coatings are described in JP-A-53-137231, 53-137233 and 53-137234 and FR-A-2636958. JP-A-53-137231 describes a polybutyral silicone rubber mixture, a polyurethane or a chloroprene rubber. JP-A-53-137233 describes a polybutyral silicone rubber mixture. JP-A-53-137234 describes a prepolymer prepared from a polyol and a diisocyanate. These undercoats are generally applied at a thickness such as 30 or 40 microns. They are used solely to promote intercoat adhesion. They have no significant effect on the abrasion and tear resistance of the silicone rubber. FR-A-2636958 describes a mixture of a chlorinated polyolefin with an amine-functional silane or its reaction product with an epoxy functional silane or an alpha, omega-dihydroxypolydimethylsiloxane oil.